The present invention relates generally to the field of plasma are welding and, in particular, to a new and useful method and device for joining two elements together using plasma arc welds such that overpenetration of the weld joint is reduced.
Plasma arc welding (PAW) is an arc welding process that produces coalescence of metal by heating with a constricted arc between an electrode and the workpiece (transferred arc) or between the electrode and the constricting nozzle (nontransferred arc). Shielding is generally obtained from the hot, ionized plasma gas issuing from the torch. This plasma gas is usually supplemented by an auxiliary source of shielding gas. The shielding gas may be a single inert gas or a mixture of inert gases. Pressure is not applied, and filler material may or may not be added.
Plasma arc welding uses a nonconsumable electrode. The torch has a nozzle that creates a gas chamber surrounding the electrode. The arc heats the gas fed into the chamber a temperature where it becomes ionized and conducts electricity. The ionized gas is defined as plasma.
One type of plasma arc weld which can be made is a keyhole weld. The keyhole plasma arc welding process is a high power density welding process, therefore the arc has the capacity to penetrate completely through a thickness of material and make welds in a single pass. In the keyhole operation, molten metal is displaced to the top bead surface by the plasma stream (as it penetrates the weld joint) to form the keyhole. As the plasma arc torch then is moved along the weld joint, metal melted at the front side of the keyhole by the arc flows around the plasma stream to the rear where the weld pool progressively solidifies. The maximum weld pool volume and the resultant underbead root surface profile are largely determined by the force balance between the surface tension of the molten weld metal, the plasma arc current, and the velocity of the ionized gas exiting the orifice. If these forces are not properly balanced, the high velocity plasma gas flow can eject the molten metal out of the weld puddle, down into the area below the weld root surface. This unacceptable condition is called overpenetration. The force balance is most difficult to balance during initiation and termination of the keyhole (e.g., at the start and stop areas of the weld).
Plates are often welded using the plasma arc weld process and a preplaced, rectangular shim of filler material. FIG. 1 shows a known shim 10 having 45.degree. tapers at each end to assist the weld operator in staring and stopping the weld process The 45.degree. angled ends of the shim 10 allow an operator approximately 3 seconds to progress from weld start parameters up to full weld thickness, solid state welding parameters as the weld moves along the 45.degree. taper. Metal is displaced during the keyhole formation to the backside of the hole and forms a large puddle with an inherent hydrostatic head being supported by surface tension and the plasma gas.
As the weld progresses along the 45.degree. taper and into the full thickness, some turbulence can occur in the puddle. The turbulence causes molten metal to be blown into the underbead area by the plasma gas flow through the keyhole, resulting in metal from the puddle sagging out, or draining, into the underbead area. This overpenetration of the metal occurs in approximately 3% of welds made with this process, leading to time-consuming and expensive repairs.